Since their first discovery several decades ago, glass-ceramic materials have found extensive use due to many unique and desirable properties. For example, glass-ceramic materials comprising β-quartz and/or β-spodumene solid solution as the predominant crystalline phases have been used in stove windows, cooktop plates, kitchen utensils, food serving devices and receptacles, and the like, for many years.
Typically, the preparation of a glass-ceramic article comprises three steps: (i) melting the precursor glass for the glass-ceramic material, which typically contains a nucleating agent, and forming the glass into the shape of the article; (ii) treating the glass article at a temperature where crystal nuclei are allowed to form in the glass; and (iii) treating the glass article at a temperature where the crystallites of the desired phases are allowed to grow to a desired extent. Steps (ii) and (iii) are typically collectively called the process of ceramming. The color of the glass-ceramic article may remain the same as that of the precursor glass, or is altered slightly, or is changed drastically, after the ceramming process.
Dark colored glass-ceramic materials (for example, in dark brown, or near black color) having low transmission in the visible range and high transmission in the infrared are popular for cooktop plates. Such cooktop plates commercially available to date typically comprise V2O5 as the coloring agent for imparting the dark tint to the glass-ceramic plate. Without intending to be bound by a particular theory, it is believed that the species of vanadium in the glass undergo color-imparting chemical reactions with other species present in the glass when the precursor glass is cerammed, resulting in chemical species that are absorbing in the visible and transmissive in the infrared.
Vanadium has been found to be a potential health hazard when the body of a mammal is subject to high-dose exposure. Currently, V2O5 is used at a very low level in the dark glass-ceramic cooktop products. There is no evidence showing that such low level of V2O5 as currently used in these products, when the products are used as intended, may cause detrimental health effects to the human body. However, being free of V2O5 or containing V2O5 at an even lower level would be a welcoming feature for such products.
Many of the current cooktop products and other glass-ceramic products available in the market today also comprise small amounts of As2O5 and/or Sb2O5 as fining agent when the glass is made. These two are widely known toxic agents. Due to their detrimental effects on human bodies and the environment in some contexts, it is highly desired that they are not present in those products, or are used at a much lower level if possible.
Hence there is the need for glass-ceramic materials comprising less or no toxic agents such as As2O5, Sb2O5 and vanadium than in conventional glass-ceramic materials. Certain embodiments of the present invention satisfy this need.